Urine concentrates from normal individuals were shown to contain interleukin 1 (IL-1)-like activity when tested directly on human dermal fibroblasts and on C3H/HeJ mouse thymocytes in the presence of 1 ug/ml phytohemagglutinin. Seventy-five percent of the urine samples tested, however, demonstrated the presence of a specific inhibitor of IL 1 promoted thymocyte proliferation. This inhibitor did not affect IL 2-promoted proliferation of mouse thymocytes or CT-6 cells or IL 1-promoted proliferation of human dermal fibroblasts. After gel filtration of the uring concentrates, even those samples that were inhibitory yielded fractions containing both thymocyte and fibroblast proliferative activity. The approximate m.w. of these activities were 75,000 and 15,000. In addition, two peaks of low m.w. thymocyte proliferative activity were noted at 4,000 and 2,000. The 2,000 pool, but not the 4,000 pool, also contained fibroblast proliferative activity. Purification methods are being developed for isolation of the urinary IL 1, to allow complete biochemical and biological characterization and for development of a screening assay to assess the clinical significance of altered levels of these substances.